In various types of sheet material handling machines, such as in folders associated with printing presses, the material is mechanically coupled to a rotating cylinder which transports the material. For instance, in pin-type folders cut sheets are led through the folder by pins associated with the cylinder which pierce the leading edge of the sheets. Such pins do not always sufficiently control the transport of the cut sheets, and may permit the corners of the sheets to lift off the cylinder and fold back, creating a "dog-ear". Such a dog-ear problem may also occur in former folders, where the open edge of the signature is leading as it is held to a conveying cylinder by grippers.
In either type of machine, the cause of the problem is the same. When the open edge of a sheet or signature is led through the folder, it is transported rapidly through relatively stationary air. The air flow with respect to the paper tends to lift the paper from the cylinder. If the pins or grippers are not located very close to the corners of the sheets or signature, the corners may be lifted and folded back, and then creased to form a hard dog-ear when the cut sheets or signature run through the next cylinder nip.
In most circumstances there is little that can be done to reposition the pins or grippers relative to the corners of the sheets or signatures. Since the tendency to form dog-ears by the above mechanism is a function of machine speed as well as pin or gripper position, the most common solution is to reduce machine speed sufficiently to reduce dog-ear formation to an acceptable level. This is an undesirable solution, however, since it substantially affects production rates.